Pre-Spray Dwell Time by Soil Class
Why this matters
The pre-spray is doing the work; the wand is mostly rinsing and recovering. Operators who rush dwell time end up scrubbing visible spots on the dry pass and selling rescue work they could have prevented with another 5 minutes of patience. CRI Seal of Approval testing measures pre-spray contact time as a primary variable, and IICRC S100 calls out dwell as a discrete cleaning step (the "T" in the TACT cleaning circle: Time, Agitation, Chemistry, Temperature). This reference matches dwell time to soil class so the field decision is repeatable.
Soil classification at the door
Classify before chemistry choice:
- Light soil. Routine maintenance carpet, vacuumed regularly, no visible traffic lanes, primarily dry particulate plus light skin oil. Most office and well kept residential carpets at a 6 to 12 month service interval.
- Medium soil. Visible traffic lane darkening, light spotting, beverage history. Typical 18 to 24 month residential interval, mid grade commercial.
- Heavy soil. Heavy traffic darkening, multiple spot history, food and pet contributions, oily soil from kitchen or shop adjacency. 36+ month residential interval or commercial that has been neglected.
- Greasy soil. Restaurant front of house, commercial kitchen periphery, automotive shop offices, hair salon. Oils and emulsifiers from food prep or product transfer dominate over particulate.
- Construction or contractor. Drywall dust, sawdust, paint overspray, plus whatever the construction crew tracked through. Different problem set, separate procedure.
Chemistry selection by soil
| Soil class | Pre-spray chemistry | pH range |
|---|---|---|
| Light | Mild prespray, low alkaline encapsulant compatible | pH 8 to 9 |
| Medium | General purpose alkaline prespray with builders | pH 9 to 10 |
| Heavy | High alkaline prespray with solvents | pH 10 to 11.5 |
| Greasy | High alkaline plus citrus or d-limonene solvent boost | pH 10 to 12 |
| Wool or stain resist nylon (any soil) | Low pH prespray (acid side or near neutral) per fiber maker guidance | pH 4 to 8 |
High alkaline chemistry on wool damages the fiber and voids the warranty. Always identify fiber before chemistry choice (see fiber id reference).
Dwell time targets
Dwell is measured from the moment the pre-spray fully wets the soil to the start of the wet pass. Visible wet is required at the end of dwell; if the chemistry has evaporated, you have no chemistry working.
| Soil class | Minimum dwell | Typical dwell |
|---|---|---|
| Light | 5 minutes | 7 to 10 minutes |
| Medium | 10 minutes | 12 to 15 minutes |
| Heavy | 15 minutes | 15 to 20 minutes |
| Greasy | 15 to 20 minutes | 20 to 30 minutes, may require re-wetting once |
| Wool with low pH | 10 minutes | 10 to 15 minutes (longer for oily soils, but never exceed fiber maker guidance) |
These targets assume ambient room temperature 65 to 75 deg F. Cold rooms slow chemistry; very hot rooms evaporate the carrier faster. Heated truck mount solution at 180 to 220 deg F at the wand has cooled to well under 100 deg F by the time it sits in pile during dwell, so room temperature is what governs working time.
Agitation during dwell
Pre-spray is more effective with mechanical agitation worked into the dwell window:
- Light to medium soil. Groomer (carpet rake style) walked over the area once after pre-spray application opens the pile and distributes chemistry through the depth.
- Heavy soil. CRB (counter rotating brush) machine after pre-spray, before dwell. Two to three passes opens pile and emulsifies soil. Reset the dwell clock after agitation since chemistry has been redistributed.
- Greasy soil. CRB or rotary with a soft pad. Heat helps. A second pre-spray application 10 minutes after the first ensures coverage of the soil that the first wave emulsified.
Re-wetting protocol
If dwell exceeds 20 minutes or you can see surface dry on the carpet pile, re-wet:
- Apply a light second mist of pre-spray over the area.
- Allow 2 to 3 minutes additional dwell before extraction.
- Adjust extraction solution rinse pH to neutralize alkaline residue (most extraction rinses are mild acid pH 4 to 6 for this reason).
Signals you did dwell wrong
- Visible spots show up after extraction in places that looked clean during cleaning: dwell was too short, pre-spray did not break the bond.
- Traffic lanes look clean wet but return to dark within 24 hours: dwell was adequate but extraction did not recover the suspended soil, often a CFM issue not a dwell issue.
- Carpet is sticky after dry: dwell was too long without re-wetting, alkaline residue left behind. Rinse pass with acid rinse fixes it.
- Carpet color appears lighter or bleached in patches: chemistry was too strong for fiber, dwell let the damage develop. Stop, identify fiber, switch chemistry.
Wand technique against soil class
Dwell does not eliminate the need for technique. After dwell:
- Wet pass at jets open, wand slow enough to fully wet pile end to end (typically 1 second per 2 to 3 linear feet)
- Two dry passes with jets closed, slow enough to recover visible water
- Repeat dry passes until the wand is recovering minimal water
- Heavy and greasy soil often need a third or fourth dry pass; budget the time
References
- IICRC S100 Standard for Professional Cleaning of Textile Floor Coverings
- CRI Seal of Approval extractor and chemistry testing protocols
- Carpet fiber manufacturer (Mohawk, Shaw, Stainmaster, WoolMark) pH and chemistry guidance
- Manuall internal: Pre-Spray Chemistry, Hot Water Extraction SOP